The present invention relates to a mechanical amplification mechanism for amplifying the motion of an electromechanical transducer element. More particularly, the invention relates to a mechanical amplification mechanism which amplifies a displacement of a piezoelectric element used as a driving source and which is suited for a printer head or a mechanical switch.
A piezoelectric transducer with a mechanical amplification mechanism has been used as a printer head of a dot-type line printer or a serial printer and has also been used as a mechanical switch, such as a relay. The printer head employing a piezoelectric transducer element as a driving source has reduced both power dissipation and the quantity of heat generation and a high-speed operation is realized. Since displacement of the piezoelectric transducer element is very slight, e.g. 0.005 mm to 0.01 mm, it should be amplified by a mechanical amplification mechanism, to an extent of 50 to 100 times, so that a sufficient printing needle stroke, e.g. 0.5 mm, may be obtained in the dot-printing head.
A piezoelectrically driven printer head is disclosed in the U.S. Pat. No. 4,193,703 entitled "Matrix Printer with Piezoelectrically Driven Printing Needles", and issued to Walter Sakmann on Mar. 18, 1980. The mechanism of this patent comprises a holding part having bent portions at both of its ends. A piezoelectric crystal is fixed, at its first end, to one of the bent portions. A buckling spring is provided between the other of the bent portions and a second end of the piezoelectric crystal. A printing needle is secured to the central portion of the buckling spring. The buckling spring is deflected by exciting the piezoelectric crystal to drive the printing needle.
In such a mechanism, the magnitude of deflection .delta. of the central portion of the buckling spring, i.e., the printing stroke, is geometrically approximated to ##EQU1## where .epsilon. denotes the elongation of the piezoelectric crystal and l the length of the buckling spring. Assuming that .epsilon.=0.01 mm, by way of example, it is not possible to make .delta.=0.5 mm unless l=60 mm. Moreover, since the elongational force of the piezoelectric crystal also acts on the holding part, the holding part is deformed to open outwardly so that the displacement to be transmitted to the buckling spring suffers a loss. After all, the length of the buckling spring should be as long as 100 mm or more. Accordingly, this structure has the disadvantage that the size of the printing mechanism becomes quite large.
To solve this problem, one of the inventors of the present invention has described a mechanical amplification mechanism which is disclosed in the U.S. patent application Ser. No. 591,981, filed on Mar. 27, 1984 and is assigned to the same assignee. The mechanism includes two lever arms respectively fixed to two movable ends of a piezoelectric element. These arms extend perpendicularly to the direction of expansion and contraction of the piezoelectric element. Free ends of those arms hold a band spring therebetween. The band spring has an acting element such as a printing needle or a switch terminal at its central portion. Each of the two lever arms is pivotably supported on a base plate by a fulcrum member located at the position between the fixed end and the free end of the lever arm. The base plate is fixed to a frame member to install the amplification mechanism. Accordingly, the two lever arms turn aorund the fulcrum members in response to an expansion of the piezoelectric element, so that their free ends approach each other. As a result, the two lever arms bend the band spring forwardly, thus causing the acting element to be driven in the direction perpendicular to the expanding direction of the piezoelectric element.
According to this mechanism, the expansion of the piezoelectric element is amplified by the two lever arms and the band spring. Therefore, a sufficient stroke, e.g., 0.6 mm, of the acting element can be obtained with the small-sized mechanism, which is necessary for a printer head and a relay.
However, the lever arms are connected to the base plate with small contacting portions defined by the fulcrum members. Moreover, the piezoelectric element and the band spring are supported by the two ends of the lever arms. Therefore, the lever arms and the band spring tends to be distorted with respect to the base plate. As a result, it is difficult to keep the actuating element in a stable and precise position with respect to the base plate which is fixed to the frame or housing. In particular, where a plurality of the printer heads are arrayed in parallel as applied to a line printer, it is difficult to array a plurality of the actuating elements in a line with high precision.
Further, the base plate is surrounded by the piezoelectric element and movable members, i.e., lever arms and band spring. It must hang in the space on a shaft-like member in order to install the mechanism. Therefore, the installation can not easily be carried out and high precision cannot be realized for the positioning adjustment of the mechanism.